The discovery of carbon nanotubes [1] triggered a worldwide research effort devoted to determining their structure [2-6], calculating and measuring their physical properties [7-15], and to improving methods of production [16-33]. Carbon nanotubes have many extraordinary physical and chemical properties. For example, the Young's modulus of multi-wall carbon nanotubes has been calculated to be up to 1.4 times that of a graphite whisker, about 1 Tpa [34]; values derived from thermal vibration experiments performed on several multi-wall carbon nanotubes in a transmission electron microscope are in the 0.4–3.7 Tpa range [35]. Moreover, their flexibility is remarkable [36] and the bending may be fully reversible up to a critical angle value as large as 110° [37].
Carbon nanotubes have many applications. For example, they can be used as supports for metal catalysts, as in the case of a ruthenium cluster (range 3–7 nm) which can be deposited on nanotube surfaces [38]. As tubular structures, they have unusual capillary properties [39]. Mechanically, nanotubes are significantly stiffer than currently commercially available carbon fibres [40], and can therefore be used to strengthen composite materials or atomic force microscope tips. Theoretical calculations of their electronic structure indicate that due to their mesoscopic structure, nanotubes may exhibit quantum effects arising from their small diameter [41]. Of high technological interest is the encapsulation of metallic particles which display physical properties such as ferromagnetism or superconductivity [42]. Filled with metals or semiconductors, nanotubes may well provide components for nanoscale electrical or electronic devices such as amplifiers, switches or electrical-mechanical converters. Carbon nanotubes have also been shown to have hydrogen storage capabilities.
Three technologies have been applied in the synthesis of carbon nanotubes. They are carbon-arc discharge, laser-ablation and catalytic decomposition processes.
In the carbon arc-discharge method, carbon-nanotubes are grown between carbon electrodes in an inert gas atmosphere [1,16]. Catalytic species such as iron or cobalt can be used during the arc-discharge to improve both the productivity and the length of tubes. However, by this process, carbon-nanotubes are obtained as a mixture with several other carbon forms, including amorphous carbon and carbon particles. Thus, purification has to be carried out and the yield of nanotubes is no more than 2% [43].
Recently discovered, laser-ablation is a new method to prepare carbon nanotubes with high yield and purity. Usually, nanosecond pulses from a Nd:YAG laser were used to ablate a target of graphite-metal composite in a inert gas atmosphere maintained at 1473K [44-47]. The presence of a transition metal or a metal alloy, together with a carbon species, is essential to form carbon nanotubes. However, with the expensive laser generator, single-walled carbon nanotubes are the main target of this method.
The above two methods were designed mainly for carbon nanotubes synthesis on a laboratory scale and were used primarily for theoretical investigation. They do not seem suitable for the large-scale production of carbon nanotubes.
The third way to make carbon nanotubes is catalytic decomposition of hydrocarbons or other organic molecules (e.g. 2-methyl-1,2-naphthyl ketone) in the presence of supported transition metal catalysts [48-51], and this method is technically based on the route developed for the production of vapour-grown carbon fibres [52]. Ivanov et al. [51], Li et al. [53] and Mukhopadhyay et al. [54] reported the production of multi-walled carbon nanotubes with 3–8 nm inner, 5–25 nm outer diameters, and up to 60–100 μm in length with remarkable efficiency at low temperature. Colomer et al. [55] have shown different methods to remove the catalyst particles and the amorphous carbon from the nanotubes samples produced by catalytic method. Since it is straightforward to scale up both the preparation and the purification method, this route seems to be the most promising one for large scale industrial applications.
During the past decade, significant progress made in fuel cell technologies has prompted the exploration of replacing traditional central large power plants with so-called distributed power generators and a membrane fuel cell [56]. The latter technology generates electricity at locations where it is to be used, and therefore eliminates the loss of electricity due to transmission. In addition, the fuel cell process does not emit any environmental pollutants such as NOx, SOx and other hydrocarbons. As a result, such a process becomes attractive for the automobile industry as well, since vehicles can then be propelled by electricity produced from an on-board fuel cell rather than by an internal combustion engine [57,58].
The current proton-exchange membrane (PEM) fuel cells utilize hydrogen gas as the energy source and require the elimination of carbon monoxide (ideally below 20 ppm) from the hydrogen stream to prevent poisoning of the electrocatalysts. Hydrogen gas is typically produced through steam reforming of methanol in vehicles [57-59] and through steam reforming, partial oxidation or autothermal reforming of natural gas for stationary uses [60,61]. In all these cases, however, carbon monoxide is a co-product, which has to be converted into carbon dioxide in subsequent steps.
An alternative route is directly cracking the hydrocarbon fuel into hydrogen and carbon. In this case, formation of carbon oxides is avoided and the need for downstream reactions such as water-gas shift and selective oxidation is eliminated. This approach has not been extensively studied, except for hydrogen production via the catalytic cracking of methane [62]. Recently, Muradov[63] studied the use of iron oxide as a catalyst for the cracking of methane and reported that equilibrium conversions were achieved at temperature above 1073K. The iron oxide also appeared to maintain its activity for several hours, in contrast to a Pt/Al2O3 catalyst which deactivated within minutes under similar conditions. Furthermore, Ishihara et al. [64,65] reported that methane cracking takes place at low temperatures over a 10% Ni/SiO2 catalyst, which does not deactivate even after approximately 200 carbon per nickel atoms have been deposited on it.